The twin towers of the World Trade Center sustained massive structural damage after two hijacked jets crashed into the 110-story buildings on Sept. 11, and the steel eventually buckled under the enormous heat generated by fires caused by spreading jet fuel, a federal team of engineers has concluded in a report to be released today.

Saying it was "remarkable" that the towers remained standing as long as they did, the engineers warned that it "may not be technically feasible" or cost-effective to design structural safeguards to protect other skyscrapers from toppling in similar attacks.

"Resources should be directed primarily to aviation and other security measures rather than to hardening buildings against airplane impact," stated the report by the Federal Emergency Management Agency and American Society of Civil Engineers.

Although the report found that the World Trade Center met building standards, it urged a national study to examine broader issues such as the quality of fireproofing materials, placement of emergency exit stairwells, evacuation procedures and firefighter response. New standards might be needed to evaluate fire resistance along with changes in building codes, the report said. Specialists also should look for feasible design changes to limit or stop collapses once they have begun, it said.

The towers, designed in the 1960s, were unique because they were built to withstand the impact of the largest jet at the time, a Boeing 707. But the design plan did not take into account the consequences of jet fuel and a massive fire, engineers said.

"There's no smoking gun where we could say there was a terrible design feature here," said Jonathan R. Barnett, a professor of fire protection engineering who helped write the report. "We went into this to study the buildings and how they performed and what lessons could be learned to make the world a safer place."

The report is the most comprehensive analysis to date of the collapse of the landmark buildings, relying on steel remnants, videotapes, photographs, witnesses and other evidence. It recounts the horrifying chronology of events, with diagrams showing how the first Boeing 767 hit the North Tower, ramming into its inner core between the 94th and 98th floors. The second plane sliced into the South Tower between the 78th and 84th floors. The South Tower fell within an hour, and the North Tower fell in 102 minutes.

More than 2,800 people died, including 343 emergency responders. Only four people who were above the point of impact when the planes hit survived. More than 25,000 people in the lower floors managed to escape, the report said, because the towers were so well-supported that they held up long enough for them to exit.

But the report found that automatic sprinklers and other fire-suppression systems were disabled by the impact of the crash, that top exit stairwells were cut off by the crash, and that much of the fireproofing meant to protect steel became dislodged. Meanwhile, "fireballs" from ignited jet fuel engulfed much of the buildings' contents.

With nothing to stop the fires from spreading, the report said, "the heat output from these fires is estimated to have been comparable to the power produced by a large commercial power generating station" and softened and weakened structural frames. The temperature of the fire reached about 2,000 degrees Fahrenheit, the report concluded.

The findings will be presented today at a hearing of the House Science Committee, which has closely monitored the review. Numerous family members of the victims plan to attend, including many who have pressed for a much more comprehensive investigation.

The larger probe is in the works, with the National Institute of Standards and Technology (NIST) awaiting $16 million from Congress to conduct a wide-ranging, two-year investigation and already mapping plans for how to proceed.

Sally Regenhard, whose son, a firefighter, died in the collapse, helped found the Skyscraper Safety Campaign and said she is "cautiously optimistic" that the investigation by NIST will help identify problems and solutions. The first study team, she said, failed to preserve much of the structural steel by allowing as much as 95 percent of it to be recycled, was denied early access to the site and lacked the power to subpoena documents and testimony.

"We're tired of waiting for a proper investigation," Regenhard said.

Rep. Sherwood L. Boehlert (R-N.Y.), the Science Committee's chairman, said at a hearing in March that no federal agency is responsible for investigating building failures, which he contended caused early confusion. Boehlert is crafting legislation that would give a new NIST agency that authority on a permanent basis, much as the National Transportation Safety Board conducts investigations into airplane and train crashes.

Rep. Anthony D. Weiner (D-N.Y.), a co-sponsor of the legislation, said it would enable NIST to subpoena witnesses and to gather and keep evidence.

The engineering team from FEMA and the American Society of Civil Engineers said it had the tools and access needed for a preliminary review. More than 70 pieces of steel have been saved for the NIST follow-up, team members said, and civil engineers are combing dump sites for pieces that could hold new clues.

One engineer, Ed DePaola, recently weaved around slopes of sliced and charred steel beams and cement wreckage languishing along the Hudson River, finally spotting one indistinguishable pile of wreckage that to him was a buried treasure -- a set of three-story beams that came from the 101st to 103rd floors of one of the towers.

Staff writer Christine Haughney contributed to this report.